1. Field of the Invention
This invention relates to a cleanliness inspection apparatus and a cleanliness inspection method in which the cleanliness of an object to be inspected is detected by measuring the quantity of particles adhering to the to-be-inspected object.
2. Description of the Related Art
A hard disk drive (HDD) is used in a data processing apparatus, such as a personal computer. The hard disk drive comprises a magnetic disk rotatable about a spindle and a carriage turnable about a pivot, etc. A disk drive suspension is mounted on an arm of the carriage. The suspension comprises a load beam and a flexure superposed on it. A magnetic head comprising a slider is mounted near the distal end of the flexure. The magnetic head is provided with elements (transducers) for accessing data, that is, for reading or writing data. As the magnetic disk rotates at high speed, an air bearing is formed between the head and a surface of the disk.
The suspension is an ultra-small precision component, and the magnetic head is located close to or at a nanometer-order distance from the recording surface of the disk. If contaminants such as particles (solid granules) are sandwiched between the disk and head, therefore, the head or the recording surface of the disk may be damaged and rendered malfunctioning.
Since the suspension, disk, etc., are accommodated in a sealed case, particles can be prevented from being intruded into the disk drive. If particles adhere to the suspension itself at the stage of its production, however, they may sometimes separate from the suspension during use of the disk drive. In addition, the separated particles may get in between the disk and head. Since the suspension is located close to the recording surface of the disk, in particular, the particles separated from the suspension may cause a head crash in some cases. Therefore, it is important to maintain the cleanliness of the suspension at a high level.
A liquid particle counter (LPC) may be used to measure the cleanliness of a component such as the suspension that requires high cleanliness. For example, a component is inserted into a liquid in an ultrasonic generator tank in which an ultrasonic vibrator is accommodated, and ultrasonic vibration is applied to the component to separate particles from its surface. The number of particles contained in the liquid is counted by means of the particle counter.
An example of the liquid particle counter is described in Jpn. Pat. Appln. KOKAI Publication No. 2009-31173 (Patent Document 1). In this liquid particle counter, various liquids (media to be measured) to be used in semiconductor manufacturing processes are contained in a quartz glass cell. Ultrasonic vibration produced by an ultrasonic vibrator propagates in the liquids. Air bubbles in the liquids are removed by ultrasonic waves propagating in the liquids, and the numbers of particles in the liquids are counted.
An example of a supercritical extraction method is described in Jpn. Pat. Appln. KOKAI Publication No. 2006-247499 (Patent Document 2). According to this supercritical extraction method, a very small quantity of a sample to be extracted is inserted into a test tube, and in addition, methanol is introduced into the test tube to fill about half of its capacity. Then, the mouth of the test tube is closed. Thereafter, the test tube is inserted into a pressure-resistant stainless-steel vessel, and methanol is introduced into the vessel. Further, the pressure-resistant vessel is sealed and heated in an oven.
After ultrasonic waves are applied to objects to be inspected in a liquid, the quantity of particles can be inspected by means of the particle counter of Patent Document 1. In this inspection, however, the number of particles determined for each object may vary, although substantially equal quantities of particles adhere to the objects. One possible cause of this phenomenon is a substantial variation in particle extraction (or variation in the ratio of separation of particles adhering to the objects to be inspected).
Let us assume, for example, that the intensity of ultrasonic waves in an ultrasonic generator tank is measured by means of a sound-pressure meter. In this measurement, the ultrasonic intensity is not stable immediately after an ultrasonic generator is powered on, so that an excessive ultrasonic intensity may be produced immediately after the start of ultrasonic oscillation. If the ultrasonic generator is powered on with a to-be-inspected object in the generator tank, therefore, the instability of the ultrasonic intensity immediately after the start of oscillation causes variation in particle extraction. Immediately after the start of oscillation of ultrasonic vibration, moreover, the ultrasonic intensity cannot be easily stabilized because of the influence of dissolved gases and air bubbles in the liquid in the ultrasonic generator tank. This also causes variation in extraction.
Thereupon, a proposal has been made to insert an object to be inspected into the liquid after powering on the ultrasonic generator and waiting for a predetermined time to stabilize the ultrasonic intensity. Ultrasonic waves that propagate from the ultrasonic generator in the liquid toward the liquid surface are 100% reflected by the interface (liquid surface) between the liquid and air. The reflected ultrasonic waves are amplified at a depth equal to half the wavelength below the liquid surface such that an area with high vibrational energy is produced near the liquid surface. Thus, the quantity of extracted particles varies as the moving speed of the to-be-inspected object and ultrasonic vibrational energy vary when the object passes near the liquid surface.
Further, a very small object to be inspected, such as the suspension, may sometimes be caused to float on the liquid surface by surface tension as it is put into the liquid. In such a case, the time before the to-be-inspected object enters the liquid varies. Thus, the quantity of extracted particles varies according to each object to be inspected.
On the other hand, the supercritical extraction method described in Patent Document 2 requires specific processing, including a process for melt-sealing the mouth of a test tube containing a sample (object to be inspected), process for sealing the pressure-resistant vessel, etc. Thus, this method takes so much time that the cleanliness of objects to be inspected cannot be speedily inspected and entails high cost.